1. Field of the Invention
The invention relates to envelope tracking (ET) radio frequency (RF) power amplifiers.
2. Description of the Related Art
Power amplifier systems incorporating envelope tracking power supplies are known in the art. Envelope tracking may be applied to Radio frequency (RF) transmitters operating across a wide range of RF frequencies, from HF (high frequency) to microwave.
In an envelope tracking RF power amplifier system, the supply voltage fed to the power amplifier is adjusted dynamically to track the envelope of the RF input signal being amplified at high instantaneous RF powers. Generating the supply voltage by tracking the signal being amplified improves efficiency of operation by providing only the necessary supply voltage to amplify the instantaneous input signal. An ‘ideal’ supply voltage instantaneously tracks the instantaneous RF input power signal such that at any instant a sufficient and only a sufficient level of voltage supply is provided.
At high instantaneous RF input powers the power amplifier operates in compression and the RF output power is primarily determined by the supply voltage rather the RF input power. This may be termed a compressed mode of operation. At low instantaneous RF input powers the supply voltage is held substantially constant at some minimum value appropriate for the power amplifier device technology. This may be termed a linear mode of operation. In the linear mode of operation the RF output power is predominantly determined by the RF input power. At medium instantaneous RF power there is a gradual transition between the compressed and linear modes of operation.
The modes of operation of the power amplifier are illustrated in FIG. 1. FIG. 1 illustrates a plot 8 of instantaneous supply voltage for the power amplifier against instantaneous RF input power to the power amplifier. The linear mode of operation takes place in a linear region denoted by reference numeral 2. The compressed mode of operation takes place in a compressed region denoted by reference numeral 6. The transition between these two modes of operation takes place in a transition region 4.
From the above it is apparent that when the power amplifier is operating in the compressed or transition regions of operation any error between the applied supply voltage and the ‘ideal’ desired supply voltage will result in an error in the instantaneous RF output power of the signal at the output of the power amplifier. The error in the output power, in turn, degrades the linearity of the transmitted RF signal resulting in increased error vector magnitude (EVM) (a measure of in-band distortion), adjacent channel leakage ratio (ACLR) distortion (close to carrier distortion) and for frequency division duplex (FDD) systems, receive band noise (far from carrier distortion).
An ‘ideal’ envelope tracking power supply (also known as an envelope amplifier or an envelope modulator) may be modelled as a modulated voltage source which is connected to the drain or collector of a power amplifier transistor via a feed network. In a typical RF power amplifier arrangement the feed network has two main functions: (i) to provide a low impedance path for DC to video frequency currents flowing between the envelope tracking power supply and the power amplifier device intrinsic collector or drain; and (ii) to provide relatively high impedance at the RF carrier frequency across which the RF carrier voltage can be developed. It can be noted that in the examples set out in this description the signals in the supply path to the power amplifier are referred to as video frequency (or high frequency) signals, and signals in the input path (i.e. the signals to be amplified) are referred to as RF signals. These two objectives for the supply feed may be opposing, particularly when the frequency separation of the highest video frequency supply currents and the power amplifier RF carrier frequency is small or non existent.
If the supply impedance—seen from the intrinsic drain or collector of the power amplifier device—at the highest frequencies at which significant video currents are present is too high, the error in the supply voltage applied to the drain/collector will result in degradation of the transmit spectrum.
It is an aim of the invention to provide an improved arrangement which addresses one or more of the above-stated problems.